The traditional magnetotelluric method, which has been widely used in studying the electric nature of the earth since 1950s, is a geophysical exploration method based on the principles of electromagnetic induction. Compared with other electric or electromagnetic methods for studying the electric nature of the earth, the magnetotelluric method has many advantages, such as the ability to measure depths, strong capability of distinguishment, narrower iso-value range, low cost, light outdoor equipment, no shield by high resistant layer, etc. These advantages, to a large extent, result from the fact that the magnetotelluric depth measurement method utilizes natural alternative electromagnetism that has large frequency bandwidth and tremendous power as field source. Built upon the equations of electromagnetic wave, the magnetotelluric depth measurement method includes scalar quantity impedance which is presumed on that planar electromagnetic waves have vertical incidence into the earth surface, and that the earth strata are in the form of horizontal stratified media. When the earth strata possess horizontal non-uniform structure and anisotropy, strata impedance varies with time and direction of the measurement, for which the tensor impendence theory under the magnetotelluric depth measurement method will be used. Many countries including China have developed various magnetotelluric depth measurement apparatuses based on the basic principles of magnetotelluric method. Also based on the magnetotelluric method is a proposed approach to interpret data from outdoor observation. Under this approach, quantitative interpretation is used to inverse delaminated layer of strata with electric nature mainly based on apparent resistivity curve. In real-time measurement, a computer that is configured to automatically inverse would be used to compute a theoretical curve using initial parameters (Hi, ρi), compare the computed theoretical curve with the actual measured curve, and based on the comparison results, continuously modify the parameters in accordance with the minimum variance theory in order to find a theoretical curve that best matches the measured curve. As a result, the parameters corresponding to this theoretical curve are the solution of inversion. Another type of inversion for the magnetotelluric method is built on a continuous one-dimensional model. Under this model, the earth resistivity is viewed as a function of the earth isotropy that varies with depths continuously. Similarly, this inversion method still starts with an initial model to compute a theoretical curve, compares this theoretical curve with the actual measured curve, modifies the initial model so as to gradually get close to the actual measured curve, and finally obtains a unique resistivity curve in which the resistivity continuously varies with depth. The magnetotelluric depth measurement method has been employed to study the earth mantle, inspect and explore sedimentary basin and earth formations containing petroleum and gas, locate and explore thermal fields, forecast earthquake, search for metal minerals and underground water source, and so forth.
Although the magnetotelluric method has been widely used, it does not offer highly precise exploration results. Under this method, oftentimes the thickness of each stratified layer is up to hundreds of meters, and sometimes even thousands of meters, and cannot be lowered down to tens of meters. Therefore, the use of this method in mineral exploration is limited for lack of sufficient exploration efficiency.